In recent years, porous material bodies such as porous metals have been intensively studied, and are in progress in the development toward practical use as filters, hydrostatic bearings, medical instruments, sporting goods and the like.
U.S. Pat. No. 5,181,549, for example, describes a process for the production of a porous body such as a porous metal. More specifically, the production process comprises dissolving hydrogen or a hydrogen-containing gas under pressure into a molten metal material, and then cooling the molten metal to solidify the same under the controlled temperature and pressure conditions.
Japanese Unexamined Patent Publication No. 10-88254 discloses a process for producing a porous metal which comprises the steps of melting a metal under a pressurized gas atmosphere and solidifying the molten metal, the metal having a eutectic point in the metal-gas phase diagram under an isobaric gas atmosphere. Japanese Unexamined Patent Publication No. 2000-104130 discloses a process for producing a porous metal body having pores controlled in shape etc., which process comprises the steps of dissolving hydrogen, oxygen, nitrogen or the like into a molten metal under a pressurized atmosphere, and cooling the molten metal to solidify it while controlling the temperature and pressure.
According to the above-described processes, a metal melted in a crucible is poured into a mold and solidified through heat dissipation from the mold. When a metal having a high thermal conductivity such as copper, magnesium or the like is employed in these processes, the molten metal is rapidly solidified through heat dissipation, so that comparatively uniform pores can be formed. However, when these processes are applied to the cases where commonly-used materials for practical use such as steels, stainless steels, etc. are used, cooling rates decrease in the inner part of metal body due to the low thermal conductivity thereof, which results in a significant formation of coarse pores, and thus it is difficult to form uniform pores. Such a porous body with uneven pore sizes is disadvantageous in that high strengths cannot be ensured because greater stresses are exerted around larger pores when a load is applied. Moreover, such a porous body cannot be used as a filter which needs uniformity of pore diameter.